1. Field of the Invention
The present invention relates to a recording and reproducing apparatus for an optical recording medium, and more specifically, it relates to an optical information recording and reproducing apparatus having a function for correcting the abnormity and/or impossibility of detection of a servo error signal.
2. Related Background Art
In the past, for example, optical discs have been known as an optical recording medium. The optical disc has concentric or spiral tracks and is divided into a plurality of sectors for permitting the recording of data having variable lengths and high speed access. The recording and reproducing of information regarding an optical disc is performed by illuminating a light beam spot emitted from an optical head onto a desired track of the disk while effecting focusing and tracking servo control.
The above servo control is generally divided into two groups, i.e., an analog servo control and a digital servo control, on the basis of the difference of signals to be handled. Recently, an interest has been directed to the digital servo control, since it can provide a stable servo control operation.
FIG. 1 shows an example of a conventional digital servo control apparatus. In FIG. 1, reference numeral 1 denotes an optical disc as a recording medium; 2 denotes an optical system for an optical head; 3 denotes a tracking error detector for detecting a tracking error signal on the basis of an output from the optical system 2; 4 denotes a focus error detector for detecting a focus error signal on the basis of an output from the optical system 2. With respect to the tracking error detector 3 and the focus error detector 4, a known detecting method used in the optical information recording and reproducing apparatus can be adopted. For example, a push-pull method can be adopted for the tracking error detection, and an astigmatism method can be adopted for the focus error detection. Incidentally, reference numerals 5 and 6 denote A/D converters for A/D converting the error signals outputted from the error detectors 3 and 4 into digital signals, respectively; 14 denotes a digital signal processing circuit; 11 and 12 denote D/A converters for converting the digital signals from the digital signal processing circuit 14 into analog signals; and 13 and 15 denote a tracking actuator and a focus actuator, respectively, for driving the optical system 2 of the optical head to predetermined directions. Next, the operation of the digital servo control apparatus having the above-mentioned arrangement will be explained.
First of all, when it is desired to record or reproduce information with respect to the optical disk 1 by means of the optical system 2, the optical system 2 is servo-controlled by the focus actuator 13 and the tracking actuator 15. This is achieved by detecting the tracking error signal and the focus error signal on the basis of the reflected light from the optical disc 1, by calculating the detected signals using predetermined exchange equations by means of the digital signal processing circuit to obtain a desired control amount, and by activating the actuators 13, 15 on the basis of the control amount.
The servo control mode can further be divided into a continuous servo control system and a sample servo control system. The former is a system for performing the servo control by detecting the tracking error signal and the focus error signal at all times during the recording and reproducing operation, and the latter is a system for performing the tracking servo control and focus servo control by detecting the tracking error signal and the focus error signal in a time shared control with the use of servo bytes provided on the fronts of the respective sectors as shown in FIGS. 2A-2C.
Next, the control with the sample servo control system by using the above-mentioned digital servo control apparatus will be explained.
FIG. 2A shows an arrangement of wobble pits and clock pits in the servo byte area used in the sample servo control system. The wobble pits serve to detect the tracking error, and the clock pits serve to generate a clock signal for reproducing the data.
In the servo byte area, when the light beam spot is in an on-track condition, outputs as shown in FIG. 2B can be obtained, as the light beam spot is moved. The tracking error signal is obtained as "0" by A/D converting the outputs and then by calculating them on the basis of the subtraction T.sub.A -T.sub.B by means of the digital signal processing circuit. If the light beam spot is slightly shifted toward the wobble pit A, as shown in FIG. 2C, the tracking error signal S is obtained by calculating the subtraction T.sub.A -T.sub.B. If the light beam spot passes across the track, the value S of T.sub.A -T.sub.B will be changed as shown in FIG. 3. The focus error signal is detected at a mirror plane (not shown) provided behind the wobble pits. Thereafter, as in the conventional method, the control amount for the tracking and focus actuators is sought by performing the calculation on the basis of the predetermined exchange equations in the digital signal processing circuit, and the actuators are activated on the basis of the control amount. In this way, the control amount for the tracking and focus actuators obtained for each servo byte area is held for a given time until a new control amount is sought in the next servo byte area. And, the control in a certain servo byte area is performed on the basis of the control amount held for that byte area.
However, in the control system as the above-mentioned sample servo control system, wherein the control is performed by detecting the tracking error signal and the focus error signal in a time shared mode, the following problem arose.
If during the error detecting operation the tracking error signal or the focus error signal is not obtained due to a defect in and/or damage to the disc or due to the presence of dust, the desired control for the light beam spot cannot be effected until the next error signals are obtained.
In order to solve such a problem, conventionally, if the error signal could not be obtained, the servo control was performed by using the error signal obtained just before the occurrence of the impossibility of detection.
However, with this conventional method, accurate control is not always obtained; particularly, if the error signal cannot be obtained continuously for a long time, the conventional method worsens the control accuracy and cannot provide an adequate countermeasure.